Cannot access pi.hole admin GUI

The interesting thing is that things used to work up until today. I had absolutely no issues with lighttpd beforehand.
I simply followed this guide, and installed pihole by using

curl -L https://install.pi-hole.net | bash

Repairing pihole does not work:

root@Alexandria:~# pihole -r

  [✓] Root user check
  [i] Existing PHP installation detected : PHP version 5.6.40-0+deb8u9
  [i] Repair option selected
  [✗] Disk space check
  [i] Unknown free disk space! 
      We were unable to determine available free disk space on this system.
      You may override this check, however, it is not recommended.
      The option '--i_do_not_follow_recommendations' can override this.
      e.g: curl -L https://install.pi-hole.net | bash /dev/stdin <option>